AR15-pattern firearms, in rifle or pistol configuration, have become extraordinarily popular, in part because of how easily they may be adapted to a variety of configurations, types of actions, and barrel/caliber combinations. A sound suppressor for this type of firearm, whether muzzle-mounted or integral in design, is typically mounted on and supported by the barrel. In pistol caliber configurations, shorter barrels (under 16 inches) are often desirable for a variety of reasons. However, under current federal law, a rifle having a barrel shorter than 16 inches with an integral suppressor typically requires two National Firearms Act (NFA) tax stamps for each transfer (one for the short barrel rifle and one for the sound suppressor).
Another possible problem with combining a relatively short barrel and integral suppressor, especially in pistol caliber configurations, is that unburned powder could accumulate in the suppressor baffles.